The invention relates to imaging of etch resistant layers, also known as xe2x80x9cresistsxe2x80x9d, in order to fabricate high resolution patterns by etching or deposition. In particular, the invention relates to the manufacturing of integrated circuits and flat panel displays and the like using an optical stepper. A stepper is an imaging device used in the semiconductor industry to project an image of a mask onto a semiconductor wafer.
Most integrated circuits today are fabricated using selective etching or deposition according to a master pattern known as a mask, using an imaging device known as a stepper. The process of fabricating high resolution patterns, mainly on planar objects, by selective etching or deposition has been well known for centuries. In general, the layer to be shaped or patterned is covered by a protective layer known as a xe2x80x9cresistxe2x80x9d. The desired shapes are created in the protective layer, usually via photo-imaging. The exposed (or unexposed, if the resist is negative working) part of the image is removed, normally by using a liquid developer to expose the layer underneath. The exposed layer can now be etched through the openings in the resist layer, which protects the covered area from the etching process. Etching can be by wet chemicals or by dry plasma (a process widely used in the semiconductor industry).
Instead of etching an additive process can be used. In an additive process a material is deposited through the openings in a resist to add to the layer underneath the resist. This deposition can be done in a wet process (as in the well known xe2x80x9cadditivexe2x80x9d process for manufacturing printed circuit boards) or in a dry process, such as a vacuum deposition by evaporation or sputtering. Another way of using a resist is in allowing chemical reactions, such as oxidation, to occur only in the areas not covered by the resist.
In general, a resist is an imagewise mask selectively controlling a chemical or physical process and limiting the process to follow the image pattern. The term xe2x80x9cresistxe2x80x9d should be interpreted in this broad sense throughout this disclosure and claims. Any other layer which has suitable properties and can be patterned by light or heat can be used as a resist. At the end of the process the remaining resist is normally removed, or xe2x80x9cstrippedxe2x80x9d.
Historically most resists were photoresists, i.e. activated and imaged by the photonic action of light. Because of this photonic action most photoresists operate in the UV part of the spectrum, where the photon energy is high. Some resists are exposed by other types of radiation, such as electron-beams. All photoresists and electron beam resists share one fundamental property: they respond to the total exposure, not to the momentary illumination. In optics, exposure is defined as the integral of illumination over time. For example, a photoresist can be exposed by 100 mW/cm2 for 1 sec to yield an exposure of 100 mJ/cm2 (100 mwxc3x971 sec) or it can be exposed by 1000 mW for 0.1 sec (100 mWxc3x970.1 sec=100 mW/cm2) with similar results. This law is also known as the xe2x80x9creciprocity lawxe2x80x9d and it is the basic law governing the exposure of photoresists.
When a certain exposure is reached, a change occurs in the resist. Since exposure is a linear function of power and time, the principles of linear superposition apply. The most common resists operate by a change of solubility in a developer.
The law of reciprocity also requires a high contrast ratio and low stray light in optical systems used to expose photoresists and electron beam resists. For example, if an exposure system has a light leakage, or stray light, of 1% (e.g.: when exposure is xe2x80x9coffxe2x80x9d, the light level does not drop to zero but only to 1% of the xe2x80x9conxe2x80x9d state) the effect of this stray light can be as large (or larger) than the main exposure if left on the photoresist for a long time.
An even larger problem is caused when trying to image high resolution features: the point spread function of the optical system causes a xe2x80x9cspreadingxe2x80x9d of light from each feature. This causes light from one feature to overlap with adjacent features and lowers the resolution. This problem is most severe in the semiconductor industry when using steppers to image a semiconductor wafer, typically a silicon wafer.
The basic elements of a stepper are shown in FIG. 1, where a mask 1, containing a pattern which has to be copied onto silicon wafer 4 is illuminated by light source 2. Mask 1 is imaged with a reduction lens 3 to form image 5, typically at a 5xc3x97 reduction. Wafer 4 is stepped by x-y positioning system 6 and 7 and each area, known as a die, is exposed with the pattern of mask 1. Typically wafer 4 is coated with photoresist before exposure, however in some cases a different layer which is capable of responding to light is used. Because of the extremely fine features (below 1 micron) in image 5 which are imaged on each die, lens 3 is not capable of fully resolving all detail without some distortion of features.
If a cross section of the image of the mask 1 is taken along line 8 it would look like graph 9 in FIG. 2. If a cross section of the same image is taken at the surface of the wafer 4 along the line 8xe2x80x2 (FIG. 1) it would look like graph 10 in FIG. 2. For further details on microlithography in general, and operation of steppers in particular, any modern book on the subject can be consulted, such as: xe2x80x9cHandbook of Microlithography, Micromachining and Microfabricationxe2x80x9d, Volume 1 and 2, Edited by P. Rai-Choudhury, SPIE Press 1997, ISBN book number 0-8194-2378-5 (V.1) and 0-8194-2379-3 (V.2).
Referring now to FIG. 2, mask 1 is transmitting light in all areas not covered by an opaque layer. The image is made of pixels (picture elements) numbered from 1 to 16 in FIG. 2. Graph 9 shows the light intensity distribution just under mask 1. After passing through the lens, this light distribution is distorted by the limited resolution of the lens (item 3 in FIG. 1). The resulting light distribution is shown in graph 10. Photoresists are formulated to have a sharp threshold. Once the exposure level crosses the threshold a chemical change occurs. The change is normally a change in the solubility of the resist in a solvent. Because of this sharp threshold a sharp image can be produced in spite of the fact that graph 10 cannot fully reproduce the details of mask 1. As long as all desired features cross the threshold an image will be formed. Line 11 represents the threshold. FIG. 2 shows the use of a positive resist, which is washed away in all exposed areas. The same theory also applies to negative resists.
The resist is exposed in all areas of image 5 where a graph 10 crossed threshold 11. In the exposed areas, the resist is washed away on the die. The features of image 5 are imaged sharply, however, their dimensions are distorted, as can be seen by comparing the pattern imaged onto wafer 4 to mask 1 in FIG. 2. For reasons of clarity the pattern imaged onto wafer 4 and mask 1 are shown at the same size, while in most cases the pattern imaged onto wafer 4 is a reduced image. For the same reasons only a one dimensional section is shown, while the same effect is happening in the other dimension as well. All graphs are shown along the X axis (as defined in FIG. 1) but the identical situation also happens in the Y axis. Also, for clarity the fact that the image on wafer 4 may be inverted (depending on the optical system used) is not shown in the graphs.
Because both the optical system and the photoresist behave as linear systems (at least as far as accumulation of exposure is concerned) the principle of linear superposition will hold. This principle states that ƒ(a+b)=ƒ(a)+ƒ(b), or the response of the system to a function made up of multiple parts is equal to the sum of the responses of the system to each part when each part is applied separately. This principle is illustrated in FIG. 3. Mask 1 can be separated into two masks, 1A and 1B each one containing only part of the image. In FIG. 3, all the transmissive pixels with an even number are placed on mask 1A, while all the odd-numbered transmissive pixels are on mask 1B. Graphs 9A and 9B generate exposure functions 10A and 10B corresponding to masks 1A and 1B. Because of the principle of linear superposition the exposures on the photoresist layer add up to graph 11, even if exposure 10A and 10B are applied sequentially and at a considerable time delay between exposures. The resultant exposure function 10 and image 5 will be identical to those shown in FIG. 2. In other words, there is nothing to be gained by breaking up the mask into multiple masks if the sum of the images equals the original image.
The difficulty of imaging features having the desired dimensions in a die on wafer 4 can be appreciated from FIG. 3. If exposure is decreased graph 10 will move down relative to threshold 11. Some features will change to become more accurate (for example, pixels 3 and 5) while others will become worse (such as pixel 11). This leaves a very narrow rang e of exposure, known as a xe2x80x9cprocess windowxe2x80x9d in which the system can be used. Even at this optimal exposure isolated clear openings become too small and wide clear openings become too wide (for positive resists).
Recently a different type of resist, known as thermoresist, has been used in the manufacturing of printing plates and printed circuit boards. A thermoresist (also known as a thermal resist or heat-mode resist) changes solubility when a certain temperature, rather than a certain accumulated exposure, has been reached. Such thermoresists are imaged using near infra-red light and therefore are also known as xe2x80x9cIR resistsxe2x80x9d. Some examples of thermoresists are disclosed in the following U.S. Pat. Nos. 4,619,894 (Bozler); 5,512,418 (Ma); 5,641,608 (Grunwald); 5,182,188 (Cole); and 5,328,811 (Brestel). Thermal resist is also available from Creo Ltd. (Lod Industrial Park, Israel), sold under the trade name xe2x80x9cDifine 4LFxe2x80x9d. All of the above-mentioned thermoresists respond to temperature and do not follow the reciprocity law.
It is not possible to have a practical true thermoresist which follows the reciprocity law. Such a thermoresist would be exposed simply by long exposure to ambient temperature just as a photoresist will g et exposed by a long exposure to ambient light). While it is possible to shield a photoresist from ambient light it is not practical to shield a thermoresist from ambient temperature. Therefore a practical thermoresist cannot obey the reciprocity law. Prolonged exposure to ambient temperatures below the threshold temperature has little effect on a thermoresist. Obviously, the threshold temperature needs to be well above the temperatures expected to be encountered in shipping and storage.
When the chemical reaction in a thermoresist does not have a sharp threshold temperature, the chemical composition is formulated to keep the reaction rate very low at room temperature. This is not difficult to do, as most chemical reaction rates approximately double every 10 degrees centigrade. Thus the reaction rate in a thermoresist exposed at 350 degrees centigrade can be a billion times faster than at 25 degrees. Using lasers it is fairly easy to raise the temperature of a thermoresist to over 1000 degrees. Such a thermoresist will appear to have a distinct threshold simply because the reaction rate at lower temperature slows down exponentially. To follow the reciprocity law the reaction rate would have to change in a linear fashion with temperature.
It is an object of the invention to increase the resolution achievable when using steppers by separating the mask into multiple masks, each mask containing only part of the information. Another object of the invention is to overcome the limitations of current steppers by using thermoresists, which can be applied in thinner layers than photoresists for increased resolution. This invention is enabled because thermoresists violate the reciprocity law. Such thermoresists do not integrate the exposure, and any stray heat dissipates quickly. It is therefore possible to image thermoresists by using multiple exposures without adding up stray light in the areas not being exposed. Separating the image into multiple exposures places fewer demands on the imaging optics since each exposure images a mask which contains fewer than all of the features to be imaged. These and other objects of the invention will become apparent by considering the following description in conjunction with the drawings.
A stepper for imaging integrated circuit and flat panel displays uses multiple masks, each one containing only part of the features which need to be imaged. The final image is generated by combining the images from all the masks on a single die taking advantage of the fact that thermal resists do not follow the reciprocity law. For maximum resolution each one of the multiple masks should contain features of only one size.